1. Field of the Invention
This invention relates to knife gate valves and in particular it relates to an improved sealing arrangement for knife gate valves.
2. Background of the Invention
Knife gate valves as contemplated herein are particularly suited for the paper processing industry for flow control of pulp and related admixtures of material suspended in a liquid sometimes referred to as a slurry. (However, knife gate valves are utilized in all types of liquid media.) The valve has a moveable steel plate referred to as a gate that in the open position is out of the flow path of the slurry. A lower edge portion of the knife gate is beveled to form an edge so that as it is closed, the gate shears through the solids of the slurry.
In one version of the knife gate valve contemplated herein, the gate valve does not have any seal other than the metal gate abutting a metal seat formed in the valve body. The design of the valve relies in part on the pressure of the slurry acting on the gate to force the gate against the seat to effect the seal.
The knife gate valve offers several advantages to a user. It has a low initial cost in comparison to other valve designs, it has a short face to face dimension so it takes up less room in the pipe line and the gate moves totally out of the flow path in the opened position.
The disadvantages of the knife gate valve are that normally the valve will seal or stop the flow of slurry in one direction only. Also, the gate of the valve by design does not provide a tight fit between the gate and the seat. Consequently, with the slurry under low pressure, increased and undesired leaking results.
In the applications where a bubble tight seal is required, (i.e. no leakage) resilient seals are incorporated into the valve. Typically, a strip of resilient material is adhered to the walls along the juncture with the gate edge when the gate is closed. The knife gate is moved into abutment with the resilient material to compress the material and effect bubble tight sealing. However, the cross sectional area of the flow path is reduced by the protruding strip to thereby reduce the flow capacity of the valve, and the seal strip, being in the path of the high pressure slurry flow are subject to rapid wear and damage, e.g. they will tear loose from the valve wall.
Other disadvantages addressed by this invention are the high cost of replacement of the valve, e.g. in the case of a damaged seal, the entire valve needs to be replaced and the need to carry an excessive inventory of the different types of gate valves, e.g. a metal-to-metal unidirectional gate valve, a bubble tight unidirectional gate valve, a bi-directional gate valve, a variable control valve, etc.